Arc welding is widely used when ships, structures and buildings such as bridges, automobiles, etc., are manufactured. In particular, in arc welding for a middle thick plate, “multi-layer overlay welding” of building up a plurality of welding beads on an object joint is used. The “multi-layer overlay welding” indicates welding to stack a layer of weld metal formed by at least one pass as defined in JIS (JIS Z 3001). A single welding operation executed along the weld joint is called pass. Also, a weld layer formed by executing the pass a plurality of times is defined as bead.
To execute the above-described multi-layer overlay welding, it is required to obtain welding conditions including the following items for each pass.    (1) Position and posture of torch: target position, target angle, advance/retract angles, etc., with respect to groove    (2) Welding output value: welding current, wire feed speed, welding voltage, etc.    (3) Torch moving method: welding speed, weaving pattern, etc.
Also, to obtain good welding quality for an object workpiece, it is required to compute proper welding conditions (1) to (3) by repeatedly executing an actual welding test in advance with use of test pieces.
Proper procedure conditions differ from one another in accordance with an object to be welded (for example, joint type (V-type, single bevel-type, T-type, etc.), plate thickness, groove angle, material of object workpiece, etc.) and a welding related device (for example, characteristics of welding power source, type of shielding gas, material, diameter, protruding length from torch tip of welding wire, etc.). Hence, the aforementioned actual welding test is required to be executed for each object to be welded and each welding device.
Also, in recent years, the welding conditions are converted into numerical values as a welding step is robotized and automated, and the numerical welding conditions are stored in a database or the like so that the numerical welding conditions can be re-used in another welding step.
However, currently, only a specific welding technician can properly derive welding conditions. Also, to obtain proper welding conditions for each object to be welded and each welding related device, it is required to execute an enormous number of actual welding tests. This is actually impossible. PTL 1 to PTL 3 disclose technologies for solving the above-described problems.
PTL 1 discloses automatic welding equipment that controls the welding speed of a welding torch unit and welds a groove part having a predetermined weld length. This automatic welding equipment includes computing means for dividing the weld length into a plurality of sections, computing the cross-sectional area of each weld layer in each section on the basis of the groove bottom surface width of the groove part, groove angle, and height of the groove part at each of a welding start portion and a welding end portion, and further computing the welding speed of the welding torch unit for each weld layer in each section in accordance with this cross-sectional area; storage means for storing information computed by the computing means; and speed control means for reading the information for each weld layer in each section from the storage means and controlling the welding speed of the welding torch unit.
That is, PTL 1 is a technology for an object to be welded being a steel-frame member for construction. The technology divides a weld cross section into respective passes, on the basis of the height, bottom surface width, and groove angle of the steel-frame member, and computes the welding speed in accordance with the deposition cross-sectional area of each pass. Also, even if the groove angle is different due to a processing error of the steel-frame member being the object workpiece, PTL 1 executes welding while setting the thickness of the weld layer to be constant by automatically adjusting the existing conditions.
PTL 2 discloses a method of executing multi-layer overlay welding for a fillet weld joint by using automatic welding equipment capable of arc welding for any joint shape by using predetermined teach data. In this method, an arithmetic processing unit that executes control on the automatic welding equipment and automatic arithmetic processing for a multi-layer overlay welding pass plan. At the automatic arithmetic operation for the multi-layer overlay welding pass plan by this arithmetic processing unit, at least the weld joint shape, welding leg length to be filled with predetermined deposition metal, gap of a joint part, and equivalent welding current, and shift amount of a welding torch from a first layer to a last layer are input as initial conditions. On the basis of this input values, the welding voltage, wire melting speed, total cross-sectional area for deposition and number of weld layers required for filling the welding leg length, number of welding passes from the first layer to the last layer, welding speed, deposition cross-sectional area per pass, bead height and width of welding in the first layer, total bead height and width obtained by built-up welding, etc., are arithmetically operated. The pass coordinates for each welding pass and the position coordinates of the welding torch from the first layer to the last layer are arithmetically operated on the basis of the arithmetic operation result, and the series of arithmetic operation results is displayed. In addition, pass plan data is created which is constituted of the optimum welding condition, the pass coordinates, and the position coordinates of the welding torch for each welding pass from the first layer to the last layer required for multi-layer overlay welding by the automatic arithmetic operation. Also, as teach data required for the fillet weld joint to be welded by multi-layer overlay welding, the welding line and the position of the welding torch for the first layer are input as initial conditions to the automatic welding equipment, and then transmitted to the arithmetic processing unit. With the teach data and the created pass plan data, teach pass plan data that determines and teaches the optimum welding line and position coordinates of the welding torch, and the optimum welding conditions for each pass from the first layer to the last layer is automatically created by the arithmetic processing unit, and then transmitted to the automatic welding equipment. Each welding pass from the first layer to the last layer is subsequently executed on the basis of this teach pass plan data.
That is, PTL 2 is a technology based on a preposition that the “equivalent welding current” is supplied in each pass for the shape, leg length, and gap length of the fillet weld joint to be welded, and the technology is for calculating welding conditions, such as the target position coordinates of the welding torch, number of passes, and welding voltage for each pass from the first layer to the last layer based on the preposition.
PTL 3 discloses a welding method in automatic welding equipment provided with a plurality of welding robots capable of simultaneous welding at a plurality of weld parts. When welding conditions are determined in accordance with the cross-sectional shape of each weld part, in a case where the welding conditions relate to the welding speed and welding current and each weld part includes a corner portion, this welding method determines the welding speed in accordance with the radial position of turning of a welding tool at the corner portion of each weld part, determines the welding speed at a straight portion before each corner portion so that start operations of turning are synchronously executed at the corner portions of the respective weld parts, and determines the welding current in accordance with the welding speed.
That is, PTL 3 is a technology that divides the total cross-sectional area of the weld part by a preset reference cross-sectional area and obtains the number of welding passes. For example, it is assumed that the reference cross-sectional area is “a cross-sectional area that can be welded by a single pass.” If a groove wider than the reference groove is used, the number of passes is automatically increased.
As described above, with the technologies from PTL 1 to PTL 3, the deposition cross-sectional area is calculated for each pass in advance, and part of the existing welding conditions is changed on the basis of each deposition cross-sectional area. Even if the shape of a joint or the like to be welded is different, new welding conditions can be automatically calculated.